1. Field of the Invention
The field of the invention is data processing, or, more specifically, a printed circuit board (‘PCB’) with reduced signal distortion, methods for signal transmission on a PCB with reduced signal distortion, and methods of designing a PCB with reduced signal distortion.
2. Description of Related Art
Common electronic equipment, such as personal computers, cellular phones, servers, and the like, often includes a number of printed circuit boards. A printed circuit board (‘PCB’) may be used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate. PCBs may include one or more layers of non-conductive substrate upon which are disposed conductive pathways and a reference plane. Such conductive pathways, referred to as ‘traces,’ may supply power to electrical circuits on the PCB or conduct signals on the printed circuit board between electrical components. A trace that conducts a signal is referred to in this specification as a ‘signal trace.’
A reference plane, also referred to as a ground plane, is a layer of conductive material, typically copper, used as an infinite ground potential on a PCB. Reference planes may include periodically recurring patterns of discontinuities. Such periodically patterned reference planes are used generally for isolating simultaneous switching noise (‘SSN’) commonly referred to as ‘ground bounce,’ reducing radiated electromagnetic interference (‘EMI’), protecting circuitry from electrostatic discharge (‘ESD’), and separating circuits of different voltage levels.
Periodically patterned reference planes of PCBs in the prior art are not oriented at an angle that reduces distortion of signals conducted on signal traces of the PCBs. In fact, periodically patterned reference planes implemented on PCBs of the prior art, typically increase signal distortion of high frequency electrical signals conducted along signal traces. Such increase of signal distortion is due, in part, to longer than necessary current return paths of high speed signals conducted on the signal traces. In addition to increasing signal distortion on PCBs, periodically pattern reference planes implemented on PCBs of the prior art also cause variations of impedance characteristics among all signal traces on a PCB, increasing difficulty of design of the PCB and electrical circuits on the PCB.
Some PCBs of the prior art are specifically configured to minimize signal distortion caused by a periodically patterned reference plan, but these configurations are time-consuming with respect to design of the PCB, extremely costly to implement, and generally ineffective. On some PCBs of the prior art, for example, signal traces are routed around discontinuities of the periodically patterned reference plane such that signal distortion caused by the orientation of the periodically patterned reference plane is reduced. Such routes of the signal traces are typically much longer than necessary, increasing cost of the PCB. Other PCBs of the prior art include an extra or very thick non-conductive substrate between the signal traces and the periodically patterned ground plane, greatly increasing the size and cost of the PCB.
Consider as one example of a PCB of the prior art, the PCB depicted in FIG. 1. FIG. 1 illustrates a printed circuit board (102) of the prior art having signal traces on the PCB (102) that are routed around discontinuities (104) of a periodically patterned reference plane (106). The PCB of FIG. 1 includes a periodically patterned reference plane (106) disposed on a surface of a layer of the PCB. The periodically patterned reference plane (116) is a conductor having discontinuities (104) arranged in a periodically recurring pattern. A segment of the pattern (116) is depicted in the example of FIG. 1 within dotted lines for ease of explanation. In this example the pattern is a grid-like pattern although readers of skill in the art will recognize that periodically patterned reference planes may include any type of periodically recurring pattern of discontinuities.
Mounted on a layer of the PCB of FIG. 1 different from the layer on which the periodically patterned plane is disposed is an integrated circuit capable of transmitting high frequency signals along any of the three signal traces (110, 112, and 114) on the PCB of FIG. 1. Each of the signal lines is routed, on a layer of the PCB different than that on which the periodically patterned plane is disposed. Each of the signal lines is also routed such that the routes cross relatively few discontinuities (104) of the periodically patterned reference plane. The route of signal trace (110) crosses discontinuities (104) of the periodically patterned reference plane only twice. The routes of signal trace (112) and signal trace (114) do not cross discontinuities (104) of the periodically patterned reference at all. Routing the signal traces (110, 112, and 114) around the discontinuities typically increases the length of such signal traces and increases design complexity as can be seen in FIG. 1.
Consider as another example of a PCB of the prior art, the PCB depicted in FIG. 2. FIG. 2 illustrates a printed circuit board (102) of the prior art in which current return paths for signals are longer than necessary. In the example of FIG. 2, the signal traces (110, 112, and 114) are not routed around discontinuities in the periodically patterned ground plane. A current return path of a high frequencies signal, such as a signal having a frequency greater than 1 gigahertz, follows a path of least inductance, a path on the reference plane that follows the signal trace on which the high frequency signal is conducted. Longer current return paths typically induce noise into a signal and distort the signal. As can be seen from the example of FIG. 2, when signal traces are not routed around discontinuities in the periodically patterned reference plane (106), the current return paths (202, 206, 204), represented in FIG. 2 by dashed lines, do not precisely follow their associated signal traces but must route around the discontinuities of the periodically patterned reference plane (106). When current return paths are routed around discontinuities, the current return path is longer than necessary, or longer than an ideal current return path. An ‘ideal’ current return path is a current return path the precisely follows its associated signal trace.
In addition to longer than ideal current return paths, the periodically patterned reference plane in the example of FIG. 2 also introduces another problem. The signal traces (110, 112, and 114) in the example of FIG. 2, even if the same length, have different impedance characteristics due to the number of discontinuities of the reference plane per unit length crossed by the signals. That is, signal traces of the same length on the PCB (102) of FIG. 2, if oriented differently, have different impedance characteristics because each of the signal traces crosses a different number of the discontinuities (104). Having different impedance characteristics for signal traces that are oriented differently increases design complexity of PCB and the electrical circuits on the PCB.
Readers of skill in the art will immediately recognize that PCBs of the prior art that include a periodically patterned reference plane have one or more shortcomings, such as for example, variations of impedance characteristics between conductive pathways, longer than necessary current return paths on the PCBs, and increased distortion of signals conducted on the PCB. What is needed, therefore, is a PCB that provides similar impedance characteristics of all conductive pathways that conduct signals on a PCB, reduces the length of current return paths on the PCB, and reduces signal distortion of signals conducted on conductive pathways of the PCB.